Normal mitotic cell division requires accurate transmission of one set of chromosomes to each daughter cell. A critical aspect of this process is the formation of a bipolar spindle with sister chromatids attached to opposite poles. Upon sister chromatid separation, the sisters are pulled apart bidirectionally to give each daughter cell a full complement of chromosomes. In fungi and somatic animal cells, the presence of two centrosomes establishes a bipolar spindle; each centrosome forms one pole. A normal cell starts the cell cycle with one centrosome, which duplicates once and only once. This process of centrosome duplication and assembly goes awry in cancer cells. Cells in tumors and carcinoma cell lines often have multiple centrosome-like structures. The presence of these extra centrosomes correlates with chromosomal abnormalities, which may underlie malignancy. Despite its central role in cellular organization, the assembly of the centrosome is understood in only the broadest of terms. The Saccharomyces cerevisiae spindle pole body (SPB) is functionally analogous to the centrosome. Unlike the centrosome, all 10 major structural components have been identified, five of which have known homologues in higher eukaryotic cells. The similarities in function and composition between the SPB and the centrosome suggests that exploiting the powerful technologies available in yeast will illuminate the process in all eukaryotic cells. The goals of this proposal are to characterize the assembly of the SPB in vivo, to reconstitute the nucleating activity of the SPB in vitro, and to study how the cell ensures assembly occurs properly.